1. Field of the Invention
The present invention relates to a controller of an internal combustion engine having intake throttle valves in intake passages of respective cylinders of the engine for adjusting intake air quantities.
2. Description of the Related Art
There has been a system having a throttle valve in an intake pipe upstream of intake manifolds of respective cylinders (i.e., in intake pipe collection part upstream of position where intake pipe branches into intake manifolds of cylinders) of an internal combustion engine for adjusting an intake air quantity and a bypass air quantity regulating valve (i.e., idle speed control valve) for adjusting a bypass air quantity flowing through a bypass passage bypassing the throttle valve to control idle speed. In such the system, there is a possibility that a leak air quantity of the throttle valve (air quantity passing through small gap between throttle valve and inner wall surface of intake passage when throttle valve is fully closed) varies due to manufacture tolerance, an aging change or the like and the controllability of the idle speed control decreases.
As a countermeasure, a device described in patent document 1 (JP-A-H5-288101) performs fuel cut control when the throttle valve is fully closed and rotation speed of the engine is equal to or higher than a predetermined value and calculates the leak air quantity of the throttle valve based on an intake air quantity sensed with an airflow meter while the bypass air quantity regulating valve is fully closed during the fuel cut control (i.e., while throttle valve is fully closed). The device controls the bypass air quantity regulating valve with the use of the leak air quantity during the idle operation.
When a device described in patent document 2 (JP-A-H9-170474) performs feedback control of the bypass air quantity regulating valve to conform actual rotation speed to target idle speed during the idle operation of the engine, the device estimates an external load of the engine and subtracts a control amount corresponding to the external load from a feedback correction amount. Thus, the device obtains and learns a value corresponding to a change of the leak air quantity of the throttle valve and corrects the feedback correction amount by using the learning value.
The applicants of the present application are currently studying a system having intake throttle valves in intake manifolds of respective cylinders of an internal combustion engine for adjusting intake air quantities. In such the system, as shown in FIG. 3, specifically in an area of a low opening degree Thr of the intake throttle valve (for example, in idle operation area), the quantity Gath of the passing air of the intake throttle valve increases and the intake air quantity increases as the leak air quantity Qleak of the intake throttle valve (air quantity passing through gap between intake throttle valve and inner wall surface of intake passage when intake throttle valve is fully closed) increases even when the opening degree Thr of the intake throttle valve is the same. Accordingly, there is a possibility that the rotation of the engine rises during the idle operation.
If the leak air quantity Qleak of the intake throttle valve decreases, the passing air quantity Gath of the intake throttle valve decreases and the intake air quantity decreases even when the opening degree Thr of the intake throttle valve is the same. Therefore, there is a possibility that the rotation of the engine falls.
Moreover, if the leak air quantity Qleak of the intake throttle valve changes, the relationship between the opening degree Thr of the intake throttle valve and the passing air quantity Gath (i.e., change characteristic of passing air quantity Gath with respect to opening degree Thr of intake throttle valve) changes. Therefore, there occurs a problem that the control accuracy of the intake air quantity by the opening degree control of the intake throttle valve lowers.
A following problem will occur if the leak air quantity of the intake throttle valve is calculated based on the intake air quantity sensed with the airflow meter while the intake throttle valve is fully closed during the fuel cut control by using the technology of the patent document 1 in the system having the intake throttle valves in the intake manifolds of the respective cylinders of the engine. That is, the capacity of the intake passage downstream of the intake throttle valve is small in the system having the intake throttle valves in the intake manifolds of the respective cylinders of the engine. Therefore, if the intake throttle valve is fully closed during the fuel cut control (i.e., when rotation speed of engine is equal to or higher than fuel cut resuming rotation speed), intake air pressure downstream of the intake throttle valve declines greatly. As a result, there is a possibility that oil loss via valve guides (i.e., phenomenon that oil lubricating sliding parts of intake valve or the like leaks toward intake port and is suctioned into intake port) occurs and the combustion state and the emission of the engine worsen.
When the feedback control of the intake throttle valve is performed to conform the actual rotation speed to the target idle speed during the idle operation of the engine with the use of the technology of the patent document 2 in the system having the intake throttle valves in the intake manifolds of the respective cylinders of the engine, a method of calculating a value corresponding to the leak air quantity of the intake throttle valve by estimating the external load of the engine and by removing the control amount corresponding to the external load from the feedback correction amount could be employed. However, it is difficult to estimate the external load of the engine with high accuracy. Therefore, the method of calculating the leak air quantity of the intake throttle valve based on the feedback correction amount and the external load has a defect that the leak air quantity of the intake throttle valve cannot be calculated with high accuracy due to an estimation error of the external load.
A system described in patent document 3 (Japanese Patent Gazette No. 2536242) has shutoff valves (i.e., throttle valves) in intake passages of respective cylinders of an internal combustion engine for adjusting intake air quantities respectively and bypass passages bypassing the shutoff valves. The system has control valves (i.e., idle speed control valves) in the bypass passages of the respective cylinders for opening/closing the bypass passages respectively. During the idle operation period, the system fully closes the shutoff valves provided in the intake passages of the respective cylinders and controls valve opening periods of the control valves provided in the bypass passages of the cylinders. Thus, the system adjusts the intake air quantities and the idle speed.
In such the system, even if the valve opening periods of the control valves provided in the bypass passages of the respective cylinders are equalized during the idle operation, a variation is caused among the intake air quantities of the cylinders if the leak air quantities of the shutoff valves provided in the intake passages of the cylinders (air quantities passing through small gaps between shutoff valves and intake passage inner walls when shutoff valves are fully closed) vary among the cylinders due to manufacture tolerances, aging changes, and the like. Therefore, there is a possibility that torque of the respective cylinders varies and the idle speed fluctuate largely.
As a countermeasure, a technology described in the patent document 3 senses the rotation speed as of expansion strokes of the respective cylinders during the idle operation and calculates average rotation speed of all the cylinders. The technology corrects the valve opening period of each control valve provided in each bypass passage of each cylinder in accordance with a difference between the rotation speed of the cylinder and the average rotation speed of all the cylinders.
This technology corrects the valve opening period of each control valve provided in the bypass passage of each cylinder during the idle operation, in which the shutoff valve provided in the intake passage of each cylinder is fully closed. Thus, the technology corrects the variation among the intake air quantities of the cylinders due to the variation among the leak air quantities of the shutoff valves of the respective cylinders or the like during the idle operation. Therefore, in the operation range, in which the shutoff valves provided in the intake passages of the cylinders are opened, the variation among the intake air quantities of the cylinders due to the variation among the leak air quantities of the shutoff valves of the cylinders cannot be corrected. Accordingly, the rotation fluctuation of the engine due to the variation among the leak air quantities of the shutoff valves of the cylinders cannot be inhibited.
Moreover, when the technology of the patent document 3 is applied, installation of the bypass passages to the intake passages of the respective cylinders and installation of the control valves in the bypass passages of the respective cylinders are necessary. Therefore, the system structure will be complicated and the cost will be increased.